Ion chemistry in the Martian dayside ionosphere

The ion composition of the Martian ionosphere is controlled by the ionisation of the neutral species (mainly CO2, CO and O) in the upper atmosphere and the chemical reactions that follow. The primary ions, CO2+ and O+, are reactive with O and CO2, respectively, as to produce O2+, which is the dominant ion species in the ionosphere. We apply a variety of simple chemical schemes to model the ion chemistry in the Martian dayside ionosphere using data from deep dip campaigns of the MAVEN mission. As model input we use concentrations of neutral species, as measured by the Neutral and Gas Ion Mass Spectrometer (NGIMS) onboard MAVEN, and solar EUV spectra measured by TIMED/SEE; extrapolated in distance and phase to Mars. We reach an adequate agreement between the calculated ion densities of the main ion species and those measured by NGIMS. However, the calculated ion composition does not fully match the measurements and deviations of up to a factor of 3-4 do prevail for some of the considered ion species. Several previous studies have solved similar issues by adjusting the input parameters to the calculations, such as increasing the neutral O density, reducing the neutral CO2 density or decreasing the solar irradiance. We present results from a thorough exploration of the involved parameter space and discuss possible reasons for still persisting model-observation discrepancies.